How to use JUnit to test asynchronous processes

An alternative is to use the CountDownLatch class.

public class DatabaseTest {    /**     * Data limit     */    private static final int DATA_LIMIT = 5;    /**     * Countdown latch     */    private CountDownLatch lock = new CountDownLatch(1);    /**     * Received data     */    private List<Data> receiveddata;    @Test    public void testDataRetrieval() throws Exception {        Database db = new MockDatabaseImpl();        db.getData(DATA_LIMIT, new DataCallback() {            @Override            public void onSuccess(List<Data> data) {                receiveddata = data;                lock.countDown();            }        });        lock.await(2000, TimeUnit.MILLISECONDS);        assertNotNull(receiveddata);        assertEquals(DATA_LIMIT, receiveddata.size());    }}

NOTE you can't just used syncronized with a regular object as a lock, as fast callbacks can release the lock before the lock's wait method is called. See this blog post by Joe Walnes.

EDIT Removed syncronized blocks around CountDownLatch thanks to comments from @jtahlborn and @Ring

You can try using the Awaitility library. It makes it easy to test the systems you're talking about.

If you use a CompletableFuture (introduced in Java 8) or a SettableFuture (from Google Guava), you can make your test finish as soon as it's done, rather than waiting a pre-set amount of time. Your test would look something like this:

CompletableFuture<String> future = new CompletableFuture<>();executorService.submit(new Runnable() {             @Override    public void run() {        future.complete("Hello World!");                    }});assertEquals("Hello World!", future.get());